Displacement electroless gold plating solutions have been used to form an intermediate layer in an effort to improve the solder adhesion of circuits, terminals, and so forth in printed wiring boards, and to improve the adhesion of reductive gold plating and the like. Most of the gold plating solutions used for this purpose contain a toxic cyanide compound as a gold compound, but concerns for the environment and the workplace require non-cyanide gold plating solutions that do not contain toxic substances.
Patent applications that have been filed for non-cyanide displacement electroless gold plating solutions include those that make use of gold sulfite compounds (see, for example, Patent Documents 1 and 2), those that make use of gold sulfites or chloroaurates (see, for example, Patent Document 3), and those that make use of gold sulfite, gold chloride, gold thiosulfate, or gold mercaptocarboxylates (see, for example, Patent Document 4). Although the electroless gold plating solutions discussed in these publications are cyanide-free, i.e. low in toxicity, and can be used close to neutral conditions, their inferior solder adhesion and film adhesion remain a problem. “Film adhesion” refers to adhesion between a displacement electroless gold plating film and a substrate and, when a displacement electroless gold plating film is used as an intermediate layer, refers to the adhesion to the layers above and below the film.    Patent Document 1: Japanese Patent No. 3,030,113    Patent Document 2: Japanese Patent Publication No. 2003-13249A    Patent Document 3: Japanese Patent Publication No. 8-291389A    Patent Document 4: Japanese Patent Publication No. 10-317157A